1. Field of Invention
The present invention is directed to golf balls, and more particularly to a multilayer golf ball having a center compression, core diameter, mantle hardness and cover hardness that provides superior playability capabilities with respect to softness and spin without sacrificing superior distance capabilities.
2. Description of Related Art
There are a number of physical properties that affect the performe of a golf ball. The core of the golf ball is the source of the ball's energy. Among other things, the core affects the ball's “feel” and its initial velocity. The “feel” is the overall sensation transmitted to the golfer through the golf ball after striking a ball. The initial velocity is the velocity at which the golf ball travels when first struck by the golf club. The initial velocity, together with the ball's trajectory, determine how far a shot will travel.
Until the late 1960's most golf balls were constructed as three-piece wound balls. In the three-piece wound ball, a solid or liquid-filled center is wound with rubber windings to form a core, which is then enclosed within a cover of compounds based on natural (balata or guttta percha) or synthetic transpolyisoprene. During the manufacturing process, after the liquid-filled center is formed, it is frozen to make it as hard as possible so that it will retain its spherical shape while the rubber thread is wrapped around it.
These three-piece wound balls were known and are still known to provide acceptable flight distance and soft feel. Additionally, due to the relative softness of the balata cover, skilled golfers are able to impart various spins on the ball in order to control the ball's flight path (e.g. “fade” or “draw”) and check characteristics upon landing on a green.
In an attempt to produce golf balls with the feel of a traditional three-piece wound golf ball various approaches were taken to duplicate the properties of balata without the materials inherent shortcomings of poor cut and shear resistance and high cost of manufacture. The first attempt at duplicating balata covers was through the use of low modulus ionomers.
These low modulus ionomer polymers produced covers with properties similar to balata but also with the inherent shortcomings of poor cut and shear resistance. Additionally these low modulus ionomer covers tended to go “out of round” quicker than traditional wound three-piece balls with balata covers. The low modulus ionomer covers were improved by blending with higher modulus ionomers but at the expense of loss of feel.
Another approach to providing a golf cover with the properties of balata without its shortcomings was described in U.S. Pat. No. 5,334,673 (the '673 patent) assigned to the Acushnet Company. The '673 patent discloses a cover composition comprising a diisocyanate, a polyol and a slow reacting polyamine curing agent.
With the advent of new materials developed through advances and experimentation in polymer chemistry, two-piece golf balls were developed. The primary difference between a two-piece golf ball and a three-piece golf ball is the elimination of the rubber thread windings found in the three-piece balls. A relatively large solid core in a two-piece ball takes the place of the relatively small center and thread windings of a three-piece ball core having the same overall diameter. With the elimination of the thread windings, there is no need to freeze the core during the manufacturing process of the two-piece golf ball.
Two-piece balls have proven to be more durable than three-piece balls when repeatedly struck with golf clubs and more durable when exposed to a variety of environmental conditions. An example of these environmental conditions is the high temperature commonly experienced in an automobile trunk. In addition, two-piece balls are typically less expensive to manufacture than the three-piece wound balls. However, two-piece balls are, in general, considered to have inferior characteristics of feel and workability when compared to three-piece balls. Generally and historically, two-piece balls use harder cover materials for increased durability. The “hardness” of a golf ball can affect the “feel” of a ball and the sound or “click” produced at contact. “Feel” is determined as the deformation (i.e. compression) of the ball under various load conditions applied across the ball's diameter. Generally, the lower the compression value, the softer the “feel.” The cores in two-piece golf balls are typically larger than the centers in three-piece golf balls.
In contrast, traditional three-piece golf balls with their smaller centers historically use softer cover materials. These softer cover materials result in a lower initial velocity when compared to two-piece golf balls. However, this difference in the initial velocity may be somewhat made up by the windings in the traditional three-piece golf ball.
Ball flight performance is also influenced by dimples formed on the cover. Dimples provide aerodynamic properties that influence flight characteristics. The dimples on a golf ball are important in reducing drag and increasing lift. Drag is the air resistance that acts on the golf ball in the opposite direction from the balls flight direction. As the ball travels through the air, the air surrounding the ball has different velocities and, thus, different pressures. The air exerts maximum pressure at the stagnation point on the front of the ball. The air then flows over the sides of the ball and has increased velocity and reduced pressure. At some point it separates from the surface of the ball, leaving a large turbulent flow area called the wake that has low pressure. The difference in the high pressure in front of the ball and the low pressure behind the ball slows the ball down. This is a primary source of drag for a golf ball.
The dimples on the ball create a turbulent boundary layer around the ball, i.e., the air in a thin layer adjacent to the ball flows in a turbulent manner. The turbulence energizes the boundary layer and helps it stay attached further around the ball to reduce the area of the wake. This greatly increases the pressure behind the ball and substantially reduces the drag.
Lift is the upward force on the ball that is created from a difference in pressure on the top of the ball to the bottom of the ball. The difference in pressure is created by a warpage in the air flow resulting from the ball's back spin. Due to the back spin, the top of the ball moves with the air flow, which delays the separation to a point further aft. Conversely, the bottom of the ball moves against the air flow, moving the separation point forward. This asymmetrical separation creates an arch in the flow pattern, requiring the air over the top of the ball to move faster, and thus have lower pressure than the air underneath the ball. In the early to mid 1900's, almost every golf ball being made had 336 dimples arranged in some form of geometrically repeating pattern. Generally, these balls had about 60% of their outer surface covered by dimples. Over the latter part of the 1900's golf balls were designed with more and more dimples in order to increase surface coverage on the ball. For example, in 1983, Acushnet introduced the TITLEIST 384, which had 384 dimples that were arranged in an icosahedral pattern. About 76% of this balls outer surface was covered with dimples.
A high degree of dimple coverage is beneficial to flight distance, but only if the dimples are of a reasonable size. Golf ball manufacturers have experimented over the years with many different dimple designs, including round, oval, truncated conical, hexagonal, etc. By varying the size, shape and volume of dimples, flight characteristics may be altered. In order to produce more desirable flight characteristics, ball designers have attempted to reduce the surface area on a ball between dimples. This surface area, commonly referred to as land area, can detrimentally effect ball performance. U.S. Pat. No. 4,142,727 to Shaw discloses a dimple pattern using between 240 to 480 dimples to achieve 50 to 60 percent dimple coverage. U.S. Pat. No. 5,957,786 to Aoyama discloses a golf ball dimple pattern based on an icosahedron design. This pattern discloses the use of between 350 to 500 dimples to cover about 80% of the balls surface. As may be seen, dimple patterns and sizes have been widely varied in order to achieve the highest possible dimple coverage on the balls surface.
Most golf balls today use relatively small dimples in order to reduce the amount of land area on the ball surface, and it is not uncommon to have golf balls with over 400 dimples on the surface. However, the only way to increase the number of dimples on a ball is to increase the number of dimple cavities in the ball mold. It is difficult and costly to design and manufacture of molds having high numbers of dimples. Thus, it would be desirable to have a ball with a high percent dimple coverage using larger size and fewer number of dimples on the ball's surface thereby avoiding complex and costly high dimple configuration mold cavities.
Ball manufacturers are bound by regulations of the United States Golf Association (USGA) which control many characteristics of the ball, including the size and weight of the ball, the initial velocity of the ball when tested under specified conditions, the overall distance the ball travels when hit under specified test conditions, and the ball's aerodynamic symmetry. Under USGA regulations, the diameter of the ball cannot be less than 1.680 inches, the weight of the ball cannot be greater than 1.620 ounces avoirdupois, the initial velocity of the ball cannot be greater than 250 feet per second when tested under specified conditions (with a maximum tolerance of +2%), the driver distance cannot exceed 280 yards when tested under specified conditions (with a test tolerance of +6%), and the ball must perform the same aerodynamically regardless of orientation.